This invention relates to processes for reducing the sulfur compound content of vacuum gas oils (VGO). More particularly, the invention relates to removing sulfur compounds that are contaminants from VGO using an ionic liquid.
VGO is a hydrocarbon fraction that may be converted into higher value hydrocarbon fractions such as diesel fuel, jet fuel, naphtha, gasoline, and other lower boiling fractions in refining processes such as hydrocracking and fluid catalytic cracking (FCC). The contaminants in VGO such as sulfur, nitrogen, metals and Conradson Carbon cause deactivation of catalysts. Total sulfur removal from feeds down to low ppm levels is an attainable goal. A significant portion of the contaminants are present as highly aromatic polar compounds. Certain phosphonium based ionic liquids have been found to selectively extract these compounds from VGO. Removal of the contaminants from the VGO will have a beneficial impact on downstream processing conditions and have an environmental impact by reducing the NOx and sulfur emissions from the regenerator. Desulfurized feeds can be further processed using noble metal catalysts. Sometimes the contaminant content of VGO feeds are reduced by hydrotreating the feed to remove nitrogen, metals and sulfur prior to further processing. However, this process uses hydrogen, in a costly process step. Additionally, hydroprocessing of feeds reduced in contaminants is significantly easier than processing highly contaminated feeds. It can be envisioned that similar aromatic compounds could be extracted from other hydrocarbon streams as well. However, VGO feed streams having higher amounts of sulfur compounds are more difficult to convert. It is the objective of the current invention to improve the ionic liquid extraction of sulfur compounds by first extracting the more highly aromatic polar nitrogen species from VGO. Experiments have shown that deep de-nitrogenation of VGO can lead to increased selectivity to sulfur removal. One approach to using this idea would be to do serial extractions on untreated VGO to first remove the polar aromatic nitrogen species and continue the operation to remove sulfur as well. Another approach would be to treat hydrotreated VGO in an attempt to remove virtually all of the polar aromatic sulfur and nitrogen species. These species are known to be refractory in hydrotreating processes and are the most likely sulfur and nitrogen species to remain after hydrotreating. This serial extraction could be done using the same ionic liquid, a sequence of different ionic liquids or by combining ionic liquids in a single extraction.
Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels. Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
Various processes using ionic liquids to remove sulfur and nitrogen compounds from hydrocarbon fractions are known. U.S. Pat. No. 7,001,504 B2 discloses a process for the removal of organosulfur compounds from hydrocarbon materials which includes contacting an ionic liquid with a hydrocarbon material to extract sulfur containing compounds into the ionic liquid. U.S. Pat. No. 7,553,406 B2 discloses a process for removing polarizable impurities from hydrocarbons and mixtures of hydrocarbons using ionic liquids as an extraction medium. U.S. Pat. No. 7,553,406 B2 also discloses that different ionic liquids show different extractive properties for different polarizable compounds. US 20110155637 discloses the removal of nitrogen compounds from vacuum gas oil by use of a VGO-immiscible phosphonium ionic liquid, but none of these processes show utility in removing refractory nitrogen compounds.
There remains a need for improved processes that enable the removal of sulfur compounds in addition to other impurities +from vacuum gas oil (VGO) either before or after hydrotreating or other treatment.